The conventional radar type underground searching apparatus has the same construction as that disclosed in the Transactions of the Institute of Electronics, Information and Communication Engineerings of '83/6 vol. J66-B No.6;713-720 in general, in which, as shown in FIG. 8, a transmitting antenna 52 and a receiving antenna 53 are arranged side by side on the ground surface 51. Pulses of the microwave range generated at a pulse generating unit 54 are transmitted into the ground 55 by way of the transmitting antenna 52 and pulses reflected by objects 56A and 56B in the ground 55 are received by the receiving antenna 53 on the ground surface 51. The pulses received are then fed to a radio-frequency amplifier unit 57 to be amplified in radio-frequency and the output from the radio-frequency amplifier 57 is utilized to detect the presence and the depth of the objects 56A and 56B.
The conventional radar type underground searching apparatus thus constructed is generally used to search for objects such as 56A or 56B which is buried under the ground at a depth ranging from several tens of centimeters to several meters. The propagation speed of radio waves (pulses) used for the search in the ground 55 is, when assuming the dielectric constant of soil is 9 for example, equivalent to 1/9.sup.1/2 =1/3 of the propagation speed in space (30 cm/ns), that is, 10 cm/ns. Accordingly, the time elapsed for radio waves to travel from the transmitting antenna 52 to the receiving antenna 53 by way of 56A or 56B will be very short such as 10 ns, 20 ns, or 40 ns when the depth to the objects is 50 cm, 1 m, or 2 m respectively. Different from a case where the radio wave travels through space, large attenuation in proportion to the exponent of the depth will take place when the radio wave travels through the ground 55, and the amount of attenuation may become as large as 1/3 to 1/10 in a depth of 1 m. Assuming the amount of attenuation is 1/10.sup.1/2 =1/3.16 in a depth of 1 m, amounts of attenuation in depths of 50 cm, 1 m, and 2 m are respectively 1/10.sup.1/2, 1/10, and 1/100 to show a great change. For example, when the reflected wave from an object buried in a depth of 1 m is attenuated to 1/10 with respect to the transmitted wave, the attenuation will become 1/100 for an object buried in a depth of 2 m. This makes it difficult to search for objects deeply buried in the ground.
To improve the low searching ability for the object 56B in the depths in the ground because of the attenuation of the radio wave and to make the search for the object 56B as effective as for the object 5A buried in a shallow depth in the ground, the conventional radar type underground searching apparatus has the radio-frequency amplifier unit 57 integrally provided with a sensitivity time control circuit. This circuit makes it possible, that the greater the time elapsed between the transmission and the reception of the radio wave becomes, that is the greater the depth of the object 56A or 56B becomes, the more the amplification of the radio-frequency amplifier unit 57 is made increased. Consequently, the difference of the object 56A and 56B in attenuation of the radio wave because of the difference in depth will be compensated so that the peaks of the reflected waves caused by the shallow object 56A and the deep object 56B are made approximately equal.
FIG. 9 shows a timing chart at every stage of the conventional radar type underground searching apparatus. The chart (A) shows a voltage waveform of pulses transmitted from the transmitting antenna 52 in which a solid line indicates a base band wave of 3 ns in pulse width and a broken line indicates a mono-pulse wave. The chart (B) shows a voltage waveform of reflected pulses fed to the radio-frequency amplifier unit 57 by way of the receiving antenna, in which a first peak P.sub.11 is caused by reflected pulses from the ground surface 51. A second peak P.sub.21 is caused by an object in a depth of 50 cm in the ground in which the reflex time t (an interval measured from the instant of zero when pulses are transmitted to the instant when the pulses are fed back to the receiving antenna 53) is 10 ns and whose amplitude is 1/10.sup.1/2 of that of the first peak P.sub.11. A third peak P.sub.31 has a reflex time of 20 ns and is caused by a object in a depth of 1 m, and whose amplitude is 1/10 of that of the first peak P.sub.11. A fourth peak P.sub.41 has a reflex time of 40 ns and is caused by an object in a depth of 1 m, and whose amplitude is 1/100, and whose amplitude is 1/100 of that of the first peak P.sub.11. The chart (C) shows a time-dependent variation of the amplification of the radio-frequency amplifier unit 57, in which the amplification varies according to the time elapsed from zero period, that is, the amplifications at t=10 ns, t=20 ns, and t=40 ns are respectively 10.sup.1/2 times, 10 times, and 100 times of the amplification at the reflex time t=0. The chart (D) shows an output voltage waveform from the radio-frequency amplifier unit 57, in which peaks P.sub.22 through P.sub.42 caused by objects in depths of 50 cm, 1 m, and 2 m respectively are approximately equal in level to a peak P.sub.12.
The reflected wave is, as mentioned above, a radio-frequency signal in the range of microwaves. It is operationally sufficient for underground search performance only to detect the presence of the peaks P.sub.22 through P.sub.42 and the time of occurrence thereof. In this connection, the conventional radar type underground searching apparatus converts the radio-frequency signal into a low frequency signal of the audio range by a low-frequency converter unit 58 whose waveforms are displayed on a waveform display unit 59 such as a synchroscope (the time base of which corresponds to the depths of the objects 56A and 56B) so that an operator can visually search for the objects 56A and 56B (to determine the presence and the location of the object 56A and 56B).
The low-frequency converter unit 58 mentioned above performs the conversion of radio-frequency signals to low-frequency signals by way of sampling, in which the reflected pulses for the transmitted pulses which are periodically transmitted are respectively sampled for one time with a delayed timing of a fixed period to obtain a low-frequency signal which is formed by extending these pulses in time base.
The sampling operation will now be described in detail by a set of timing charts in FIG. 10. The chart (A) shows a voltage waveform of pulses periodically transmitted, (B) shows a voltage waveform of reflected, pulses corresponding to the pulse of the chart (A), and (C) shows a part of low-frequency signal in one reflected pulse of the waveform shown in the chart (B) extended in time base. In FIG. 10, the pulses are repeatedly transmitted at a period of T and the reflected pulses occur at the corresponding period. Assuming the sampling period of the reflected pulses is T+.DELTA.T in this case, the sampling is performed at a series of periods of t=0, T+.DELTA.T, 2(T+ ), 3(T+.DELTA.T), and so on. When considered with respect to the transmitting period as a reference the sampling for the first reflected pulses is performed at t'=0, the second reflected pulses at t'=.DELTA.T, the third reflected pulses at t'=2.DELTA.T, and the fourth reflected pulses at t'=3 T. As a result, if each reflected wave is identical in waveform to each other, the same sampling data as taken by sampling one reflect wave at every .DELTA.T is obtained at every t+.DELTA.T. By feeding this sampling data to a low-pass filter, the low-frequency signal in FIG. 10(C), which is produced by extending one reflected pulse in the waveform in FIG. 10(B) (T+.DELTA.T)/.DELTA.T times, can be obtained.
The above mentioned procedure will now be described giving actual figures. The period during which the sampling of the reflected pulses searching period is performed depends upon the dielectric constant of the soil concerned. When the searching is performed for a depth of approximately 5 m in the ground whose soil has the dielectric constant of 9, the period may be from the transmission of pulses to the time when reflex time of 100 ns is reached. Assuming that pulses are transmitted at a period of 50 .mu.s and the number of samples during the searching period is 1000, the pulses should be transmitted 1000 times to obtain one low-frequency signal, which requires the searching period of 50 ms (=50 .mu.s.times.999+100 ns). In this case, a reflected pulse of 100 ns in length is converted into a low-frequency signal of 50 ms in length so that the period at which the low-frequency signal is produced may be set to 50 ms or more.
The following table shows the relationship between the pulse number and the sample timing.
TABLE 1 ______________________________________ Pulse number Sample timing ______________________________________ 0 0 sec ##STR1## 2 ##STR2## i ##STR3## 999 ##STR4## ______________________________________
In FIG. 8, the reference numeral 60 indicates a control unit which controls the operational timing of the pulse generator unit 54, radio-frequency amplifier unit 57, and low-frequency converter unit 58. Antennas for use with underground searching systems such as used in the conventional radar-type underground searching apparatus as the transmitting antenna 52 and the receiving antenna 53 will now be described. Antennas of this kind are required to have a return loss characteristic which is flat in the range of 50 MHz to 400 MHz and free of ringing effect in general.
The antenna described herein has approximately the same structure as the one disclosed in Japanese Patent Publication No. 044916 of 1980 and is provided with a pair of antenna elements 71 and 72 of identical flat plates having a shape of an acute isosceles triangle which are symmetrically arranged in a plane with their vertexes 71a and 72a abutted to each other as shown in FIG. 11.
The antenna elements 71 and 72 are respectively provided with at their both ends strip-shaped conductors 77 and 78 as well as 79 and 80. The conductors 77 and 78 are respectively arranged with their one ends spaced from both base ends 71b and 71c of the antenna element 71 about 1 cm and the conductors 79 and 80 are respectively arranged with their one ends spaced from both base ends 72b and 72c of the antenna element 72 about 1 cm. The other ends of the conductors 79, 80 and the other ends of the conductors 77, 78 are spaced about 1 cm apart. Further, load resistors of about 150 .OMEGA. 73 through 76, 81 and 82 are respectively connected between the base end 71b and the one end of the conductor 77 as well as 71c and 78; between the base end 72b and the one end of the conductor 79 as well as 72c and 80; between the other end of the conductor 77 and the other end of the conductor 79 as well as the other end of the conductor 78 and the other end of the conductor 80.
The vertexes 71a and 72b of the pair of the antenna elements 71 and 72 are feeding points to which the pulse generating unit (base band pulser) 54 is connected through a balun 53, and pulses from the pulse generating unit 54 are then transmitted through the pair of the antenna elements 71 and 72 as radio waves.
The vertexes 71a and 72a are fixed to an insulated balun case which contains the balun 83 and a bottom opened housing 85 made of aluminum and lined with ferrite encloses the assembly including the pair of the antenna elements 71 and 72 together with the balun case 85. In this case, the plane in which the pair of the antenna elements 71 and 72 is arranged is made flush with the plane of the opening surface of the housing 85. The pair of the antenna elements 71 and 72 is connected to the pulse generating unit 54 (see FIG. 8) in a circuit block 91 mounted on the rear side of the housing 85 through a connector 86 provided on the balun case 84, a coaxial cable 87, a connector 88, a coaxial cable 89, and a connector 90. The transmitting output from the pulse generating unit 54 in the circuit block 91 then fed to the pair of the antenna elements 71 and 72 through the connector 90, the coaxial cable 89, the connector 88, the coaxial cable 87, the connector 86, and the balun 83. Pulses are then finally transmitted from this pair of the antenna elements 71 and 72.
The conventional radar type underground searching apparatus mentioned above has such a drawback that it is necessary for the ordinary operation to increase the amplification of the radio-frequency amplifier unit 57 10 to 100 times within a period of several tens of nano-seconds, which causes distortion including the production oscillations on the reflected pulses to result in a limited improvement in searching ability and resolution.
On the other hand, when the sensitivity time control is performed in the range of low-frequency signals, that is, when it is performed after the sampling operation, there is another problem to occur. To convert radio-frequency signals it is necessary to perform sampling as mentioned above, and in this conversion there occurs inherently a sampling noise of about several millivolts. Assuming that there is a sampling noise of 5 mV, it is amplified to 50 mV when the amplification in the low-frequency range is 10, and frequency range is 10, and it is amplified to 500 mV when the amplification is 100. That is, the more the amplification is increased to amplify weak reflected pulses from the depths in the ground, the greater the sampling noise becomes, which prevents the searching ability from getting improved.
The antenna of the conventional underground searching apparatus has a wide, omnidirectional directivity, which is not suitable for the performance of searching for objects buried in the ground. That is, when the antenna is used as a transmitting one, there is a lot of leakage radio waves dissipated into space and as a result, an effective radio wave transmission into the ground will not be expected. In addition, the radio wave transmitted is widely scattered in the ground before it is reflected back to the receiving antenna, which prevents the detection accuracy for buried objects from getting improved. When the antenna of this kind is used as a receiving antenna, it accepts noise signals through space and as well as reflected waves from a number of objects scattered over a wide area in the ground. This also contributes to lower the deflection accuracy for buried objects.
A first object of the present invention is, therefore, to provide a radar type underground searching apparatus in which the searching ability and resolution can be improved.
A second object of the present invention is to provide an antenna for ground searching having a narrow directivity by which the detection accuracy for buried objects can be improved.